Symbiotic Optimization of the Nanolithography and RF-Plasma Etching for Fabricating High-Quality Light-Sensitive Superconductors on the 50 nm Scale

We present results of a fabrication-process development for the lithographic pattern transfer into the sub-100nm range by combining electron-beam lithography and reactive dry etching to obtain high quality niobium-based light-sensitive superconducting devices. To achieve this spatial resolution, we systematically investigated the stability of the positive organic etching masks ZEP 520A and PMMA 950k in different properly operated fluoride based plasma discharges. The chemically more robust ZEP 520A was used for defining the nanoscaled superconductors during the dry plasma etching. Our etching recipe is appropriate for a precisely controlled removal of a number of transition metals, their nitrides and a number of lithographic resists. Our process yielded lightsensitive superconducting devices made from NbN with smallest planar lateral dimensions of about 50nm with a critical temperature Tc(0) of about 13K , which is close to the transition temperature of the unstructured thin film. Our ultra-narrow current paths are able to permanently carry bias-currents up to 60% of the theoretical de-pairing current-limit.